Engine oil and filter on the AC Light Six

The grade

The factory specified a monograde SAE 30, with SAE 40 in climates above about 32 degrees. The engine has white metal bearings rather than modern shells, which is the fact underneath the whole discussion.

The AC two litre community takes an unusually firm line against modern oils, stating that it cannot be stressed too strongly that modern high detergent oils, and especially multigrades, should not be used in an old design of engine, on the grounds of dislodged sludge, increased consumption, ring sticking and poor drain down when parked. The archive reports that as one strongly held owner position rather than as a factory instruction, because that is what it is.

Against it stands near universal modern practice, which is a 20W/50 classic multigrade in engines of this era. The archive could find no AC specific endorsement of either position from the club or from a specialist, so it is not going to pretend the question is settled. What it will not do is present 20W/50 as sourced for this engine, because it is not.

Oil pressure, and why a low reading is probably fine

Period service data for the 1954 to 1955 Ace and Aceca gives hot oil pressure as 60 to 80 pounds per square inch above 40 miles per hour. Owners report seeing as little as 10 to 15 pounds at a warm idle, rising to 80 at higher revolutions.

That low hot idle figure is normal on this engine. It is worth saying loudly, because an owner arriving from a modern car reads 12 pounds at idle as a failure and starts dismantling. The number that matters is what it makes at speed and whether it is stable, not what the needle does when the engine is hot and doing nothing.

The filter, and what it does not do

The AC engine was not originally fitted with a full flow oil filter. Early engines had only a gauze strainer to keep large debris out of the oilways. From around 1951 or 1952 a Fram bypass filter appeared, mounted on the exhaust manifold, with the element renewed every 10,000 miles, the element being a Fram C3 or the Greyfriars 530P equivalent.

A bypass filter cleans a proportion of the oil in circulation rather than all the oil going to the bearings. That is not a fault, it is how the engine was designed, but it is worth understanding before deciding how often to change the oil.

The filter itself sits in the sump alongside the oil pump, the pressure relief valve and the starter motor. The specialist's own description of the original arrangement is that it is not very efficient and is difficult and messy to change. A remote spin on conversion is available for all models and exists precisely to solve that, and it is the single most useful modification on this page.

Zinc, and why the usual argument does not apply here

Owners of classic engines are told repeatedly that modern low zinc oils will destroy their camshaft. That argument is about the sliding boundary contact between a cam lobe and a flat tappet, and it is a real problem on flat tappet engines.

The AC Light Six does not have flat tappets. Its overhead camshaft acts on rockers whose cam contact face is a roller running on a pin, and the specialist who rebuilds these engines, AC6, sells rocker rollers and pins as a routine wear item. A rolling contact is not the interface the zinc argument is about.

The archive is being careful here about what is fact and what is inference. The roller follower is sourced. The conclusion that the flat tappet zinc case does not transfer follows from it, and general guidance agrees that roller cam engines do not have the flat tappet problem, but no AC specific recommendation on zinc was found in any club or specialist source. What is documented is where this valve gear actually wears: rocker rollers and pins wearing badly, and rocker shafts wearing their bores oval. Those are mechanical wear items rather than problems an additive solves.

Intervals

These come from the AC two litre saloon schedule, the engine being the same unit. No Ace or Aceca specific schedule was found. They read as short by modern standards and they are, but a 2,000 mile oil change on an engine with a bypass filter and white metal bearings is not the overcaution it looks like.

Working on an alloy block

Fasteners are British Standard Fine, so do not reach for metric or UNF. Sump bolts are three eighths BSF.

Torque figures, all from the saloon and all from a single source, so treat them as a guide and confirm before relying on them: cylinder head 40 pounds feet, main bearing caps 40, big end bolts 25, flywheel 28. The head is tightened working outwards from the centre pair of nuts, leaving the four longer studs until last, then re-tightened after warm up and again after 100 miles.

Forty pounds feet on a cylinder head is modest, and that is the point. Iron engine habits strip threads in an alloy block. Thread inserts in this engine are routine rather than a bodge: the specialist fits them as a matter of course when replacing water jackets and offers reforming of stripped threads as a standard service.

Related reading

• Cooling the AC Light SixCoolant choice, the water pump fault and reading the gauge.
• Servicing hubWhere each task fits in a full service.
• The AC Light SixBackground to AC's own two litre six.
• Parts and specialistsWho supplies parts for this engine, AC6 among them.

This guide is not a substitute for the official AC Workshop Manual. If anything here is wrong or can be improved, please get in touch.